mysql - 获取每行两列之间的最小值

Question

这是我的数据表:

| uid |   date   | visit | transactionDate |
+-----+----------+-------+-----------------+
|  1  | 6/2/2014 |   1   |     6/9/2014    |
|  1  | 6/2/2014 |   1   |     8/4/2014    |
|  2  | 6/2/2014 |   1   |     8/2/2014    |
|  2  | 6/2/2014 |   1   |     10/17/2014  |
|  2  | 6/2/2014 |   1   |     10/20/2014  |
|  3  | 6/2/2014 |   1   |     6/9/2014    |
|  3  | 6/2/2014 |   1   |     6/10/2014   |
|  3  | 6/2/2014 |   1   |     6/11/2014   | 
|  3  | 6/2/2014 |   1   |     6/12/2014   |
|  3  | 6/2/2014 |   1   |     6/14/2014   |
|  3  | 6/2/2014 |   1   |     6/15/2014   |
|  3  | 6/2/2014 |   1   |     6/17/2014   |
|  3  | 6/2/2014 |   1   |     6/18/2014   |
|  3  | 6/2/2014 |   1   |     6/23/2014   |

我正在尝试编写一个查询来提取两列日期和交易日期中的最小值。有没有办法做类似 MIN(date, transactionDate) 的事情？查询应选择如下内容:

uid 1 then minimum of date and transaction_dt
uid 2 then min date and transaction_dt

Answer 1

使用CASE条件。

SELECT uid, visit, 
   CASE WHEN date < transactionDate THEN date ELSE transactionDate END AS minDate
FROM table;